Transparent conductive films have high transmittance and high conductivity in the visible light range, and are used in electrodes of liquid crystal display units or various light receiving elements such as solar cells, while they are also widely used in thermic ray reflective films or antistatic films for automobiles or construction materials, and anti-fogging transparent heating units in freezer showcases and the like.
Such transparent conductive films that are known include indium oxide films that contain tin as the dopant, indium oxide films that contain zinc as the dopant, and zinc oxide films that contain one or more elements of Group III of the Periodic Table as dopants.
Indium oxide films that contain tin as the dopant are known as ITO films, and they easily yield low resistance films. However, because the indium in ITO films is an expensive rare metal, there has been a limit to cost reduction when such films are used. Furthermore, because indium reserves are low and can only be obtained as a by-product of zinc mineral processing, it is currently difficult to achieve significant production increase and stable supply for ITO films.
Indium oxide films containing zinc as a dopant are known as IZO films and are films exhibiting excellent low resistance, but the problems associated with the indium starting material are the same as for ITO films.
Consequently, avid efforts are being made to develop a material for transparent conductive films as a substitute for ITO. Among them, zinc oxide films that comprise zinc oxide as the main component and contain elements of Group III of the Periodic Table, due to the very low cost of the main starting material zinc and the high abundance of its reserves and product, are raising interest as they do not entail concerns regarding resource depletion and stability of supply as with ITO films, while they are also inexpensive and chemically stable, and exhibit excellent transparency and conductivity as well (see Patent document 1, for example).
Incidentally, zinc oxide (ZnO) is an oxide semiconductor, and intrinsic defects such as oxygen vacancies, due to deviations from the stoichiometric composition, form a donor level such that n-type properties are exhibited. When the zinc oxide contains an element of Group III of the Periodic Table, the conduction electrons are increased and resistivity is reduced. As elements of Group III of the Periodic Table included in zinc oxide there are known aluminum (see Patent document 1 and Patent document 2, for example), gallium (see Patent document 3, for example) and boron (see Patent document 4, for example).
When known zinc oxide-based sputtering targets of the prior art are used as means for forming thin-films such as transparent conductive films, anomalous discharge that occurs during sputtering results in problems such as a reduced availability factor of the sputtering apparatus and lower product yield due to the effect of generated particles.
As means for inhibiting anomalous discharge generated during sputtering there has been proposed, in Patent document 1 for example, an inhibiting effect by high-densification of the sintered compacts through modification of the production method. It has also been shown that anomalous discharge can be further inhibited by, for example, densifying the sintered compact and limiting to no greater than 5 μm the aluminum component aggregation diameter attributed to the aluminum oxide that has been added as the oxide of an element of Group III of the Periodic Table (see Patent document 5).
It has additionally been shown that if the mean particle size of ZnAl2O4 particles composed of zinc oxide and the aluminum oxide additive is limited to no greater than 0.5 μm, anomalous discharge during sputtering is inhibited and the production yield of thin-films with improved humidity resistance is increased (see Patent document 6).